1. Field of the Invention
The present invention relates to compositions and methods useful in the study, prevention and treatment of a variety of diseases characterized by a diminished capacity to regulate smooth muscle function. More particularly, the invention relates to compositions and methods for delivery of elastin-based compositions including elastic fibers, elastin, tropoelastin, or fragments thereof. Preferred compositions and methods of the invention are useful in the prophylaxis treatment of vascular disease.
2. Description of the Related Art
Vasculogenesis begins early in vertebrate development and culminates in the formation of a complex network of arteries, veins, and capillaries. Once formed, the gross and microscopic structure of this network is stable unless disrupted by disease. Genetic and cell culture studies have begun to identify molecular determinants of vasculogenesis, and these determinants have defined three distinct stages of vascular development (Hanahan, 1997; Folkman and D'Amore, 1996). In the first stage, splanchnic mesoderm coalesces to form simple tubes of endothelial cells. Vascular endothelial growth factor defines this stage (Shalaby et al., 1995; Fong et al., 1995; Carmeliet et al., 1996). The second stage involves the recruitment of mesenchymal cells by the endothelium, a process coordinated by angiopoietin and platelet-derived growth factor (Ferrara et al., 1996; Suri et al., 1996; Lindahl et al., 1997; Sato et al., 1995). In the third stage, mesenchyme differentiates into smooth muscle and extracellular matrix deposition begins. Transforming growth factor beta has been implicated in this stage (Folkman and D'Amore, 1996; Beck and D'Amore, 1997). After the third stage of vascular development, arterial smooth muscle cells exit the cell cycle and vascular structure is stabilized (Schwartz et al., 1990; Owens, 1995; Glukhova et al., 1991).
There is growing evidence that the extracellular matrix regulates cellular function during organogenesis. Fibronectin, vitronectin, collagen, and other extracellular matrix proteins bind to integrins on the surface of cells (Gumbiner, 1996; Hynes, 1992), providing morphogenic signals that regulate cell proliferation, migration, and differentiation (Adams and Watt, 1993; Hynes, 1994). Disruption of fibronectin in mice causes dramatic developmental abnormalities, including failure to develop a notochord and somites (George et al., 1993). Null mutations in genes encoding fibronectin receptors, or integrins, lead to embryonic or perinatal death from developmental abnormalities resembling those observed in mice lacking fibronectin (Yang et al., 1993; Yang et al., 1995). Not all cell-matrix interactions, however, are necessary for normal morphogenesis. For example, disruption of vitronectin, tenascin C, and integrin alpha 1 have no apparent effect on development (Zheng et al., 1995; Saga et al., 1992; Gardner et al., 1996).
Elastin is the dominant arterial extracellular matrix protein (Parks et al., 1993). This protein is encoded by a single gene and organized into polymers that form concentric rings of elastic lamellae around the arterial lumen. Each elastic lamella alternates with a ring of smooth muscle, forming a lamellar unit. The function of elastic fibers was thought to be structural, providing tensile strength and resiliency to the aorta and other arteries. Because of its structural role, investigators believed that disruption of elastin would lead to dissection of arteries. This view was supported by studies associating decreased elastin content and increased elastase activity with arterial aneurysms in humans and other species (Thompson, 1996; Terpin and Roach, 1987). In addition, disruption of collagen I and fibrillin, prominent arterial extracellular matrix proteins, resulted in rupture of blood vessels in mice and humans (Lohler et al., 1984; Dietz and Pyeritz, 1995). Human molecular genetic studies demonstrated, however, that ELN mutations do not cause arterial dilatation, but instead cause an obstructive arterial disease, supravalvular aortic stenosis (SVAS) (Curran et al., 1993; Ewart et al., 1993).
Obstructive vascular diseases cause over 40% of mortality in the United States. Vascular obstructive pathology consists of an accumulation of vascular smooth muscle cells and matrix components in the subendothelial space that occludes arterial lumens and restricts blood flow. Monocytes and macrophages are activated and release growth factors and cytokines at the site of injury. These peptides induce vascular smooth muscle cells to dedifferentiate and lose their contractile phenotype, migrate, proliferate, and occlude blood vessels.
Conventional therapies focus mainly on reducing the risk factors associated with obstructive vascular disease. Anti-thrombotic, anti-hypertensive, and cholesterol-lowering medications are aimed at decreasing the risk of occlusion, while beta blockers and angiotensin converting enzyme inhibitors act by reducing the workload of the heart.
The treatment of vascular stenosis remains surgical, consisting of bypass grafting and angioplasty. Over 600,000 bypass grafts are performed annually at an average cost of $45,000 per procedure. This procedure uses native vessels from the legs (saphenous veins) and breastbone (internal mammary arteries) to bypass the occluded vessels of the heart. Because bypass grafting entails open heart surgery, angioplasties have become the first line of treatment for obstructive vascular diseases. In the United States alone, this procedure is performed more than 660,000 times per year at an average cost of $20,000 per procedure. Briefly, this technique involves visualizing the obstructing lesion through angiography and relieving the obstruction by placing an intravascular balloon across the lesion and expanding it.
Unfortunately, only 60–70% of angioplasties lead to long-term (6 months) relief of arterial obstruction. In a process termed restenosis, the vascular smooth muscle cells differentiate, proliferate, and reocclude the artery following instrumentation. Many strategies have been employed to reduce and prevent vascular restenosis. However, a variety of methods of relieving the initial vascular obstruction including balloon angioplasty, atherectomy, and rotablation have not reduced the incidence of vascular restenosis. The use of metal intravascular stents in conjunction with either radioactivity or synthetic polymers has been similarly ineffective. Finally, attempts to inhibit vascular smooth muscle cell proliferation and migration using growth factors and cytokines have also not proved successful. Thus, there remains a significant need for compositions and methods useful in the prophylaxis and treatment of obstructive vascular disease.